Method of electrolytic deposition of an intrinsically conductive polymer upon a non-conductive substrate

ABSTRACT

A capacitor and method for manufacturing the capacitor. The capacitor comprises an anode; a dielectric oxide layer coated on the anode and a plurality of conductive islands coated on the dielectric oxide layer. An organic conductive cathode is coated on the dielectric layer and conductive islands.

BACKGROUND OF THE INVENTION

The present invention relates to an improved method of electrolyticdeposition of a conductive polymer onto a non-conductive substrate. Morespecifically, the present invention relates to an aqueous solutioncomprising monomer and surfactants whereby a conductive polymer can bedeposited on a non-conductive substrate efficiently without thenecessity for a continuous seed layer.

Solid state electrolytic capacitors are well known in the art. Thecapacitors typically comprise an anode, or positive electrode, made froma porous valve-action metal. A solid electrolyte forms a cathode, ornegative electrode. An oxide film, typically formed as the oxide of theanode, acts as a dielectric between the cathode and anode. Othercomponents are typically included such as leads, encasement layers andthe like.

Manganese dioxide has been employed in the past as a solid stateconductor in capacitors for many years. The tendency of manganesedioxide to support ignition upon failure of the capacitor has ledinvestigators to search for less powerful oxidizing agents which can beused as solid state conductors. This research has led to the widespreaduse of conductive polymers such as polypyrroles, polyanilines andpolythiophenes.

Electrodeposition of a monomer onto an oxide surface, and electrolyticpolymerization thereof has been difficult due, in part, to the inherentinability of an oxide layer to conduct. To overcome these problems ithas been standard practice in the art to form a continuous seed layer ofa suitable conductor, such as manganese dioxide, on the non-conductiveoxide layer prior to electrodeposition of the intrinsically conductivepolymer. A bothersome requirement for this process is the necessity fora continuous deposit of the manganese dioxide layer. Where voids existin the manganese dioxide layer the electrodeposition is insufficientthereby resulting in poor coverage of polymer and failed capacitors. Thedesire to decrease manufacturing losses therefore led those skilled inthe art to err on the side of a thicker manganese dioxide coating toavoid discontinuities in the seed layer. This increases both the amountof material required and the manufacturing complexity. The thickermanganese dioxide is a manufacturing necessity which does not enhancethe product as viewed by the end user. It is therefore a desire todecrease the amount of the manganese dioxide seed layer withoutcompromising the manufacturing yields or product performance. Thesedesires have been contradictory prior to the present invention.

The present invention circumvents the problems in the art by providing atechnique for electrodeposition whereby the conductive polymer can beadequately deposited on the surface of the dielectric and theelectrodeposited conducted polymer can be deposited on a discontinuousseed layer. This advance greatly increases manufacturing efficiency byallowing lower seed layer loading on the surface of the dielectric.

BRIEF SUMMARY OF THE INVENTION

It is object of the present invention to provide a highly efficientmethod of forming a capacitor.

It is another object of the present invention to provide a method forelectrodepositing conductive polymer onto a non-conductive layercomprising a discontinuous seed layer, such as manganese dioxide,deposited thereon.

A particular feature of the present invention is the ability to form acapacitor with less manganese dioxide on the surface of the dielectricthereby decreasing the material demand and the manufacturingdifficulties associated with insuring deposition of a continuous layerof manganese dioxide or seed layer.

Another particular feature is the ability to utilize existingmanufacturing systems with minor, or no, manufacturing modifications.

A particular feature of the present invention is the increasedmanufacturing yield which can be obtained by removing the need todispose of those elements comprising discontinuous manganese dioxidelayers. Prior to the present invention material with a discontinuouslayer of manganese dioxide would be considered unsuitable whereas withthe present invention they are suitable for use. With the presentinvention, an oxide layer comprising a discontinuous manganese dioxidelayer is still properly coated with the conductive cathode layer.

A particular feature of the present invention is the ability to utilizeessentially alcohol-free solvents for deposition of the conductivepolymer without detrimental performance or loss in manufacturingefficiency. Alcohol reduction is an ongoing effort in all industries asis well known.

These and other advantages, as would be realised to one of ordinaryskill in the art, are provided in a method for forming a capacitor. Themethod comprises forming an oxide layer on a valve metal. The oxidelayer is contacted with a solution comprising a monomer precursor of thepolymer of Formula I:

wherein:

-   X is S, Se or N;-   R¹ and R² are not hydrogen;-   and a compound of Formula II:    R⁴—SO₃Na  FORMULA II    wherein R⁴ is a C₃-C₂₀ linear or branched alkyl. The monomer is then    polymerized.

Another embodiment is provided in a method for forming a capacitor. Themethod comprises forming a valve metal into a shape to form an anode.The anode is contacted with an oxidizing solution to form a dielectriclayer on the anode. A discontinuous seed layer is formed on thedielectric layer to form a mixed oxide surface. The mixed oxide surfaceis contacted with an aqueous solution comprising a monomer precursor tothe polymer of Formula I wherein R¹ and R² independendly represent —OR³wherein R³ represents hydrogen, linear or branched C₁-C₁₆ alkyl orC₂-C₁₈ alkoyxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzyl which areunsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy or halogen; orR¹ and R², taken together, are linear C₁-C₆ alkylene which isunsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen,C₃-C₈ cycloalkyl, phenyl, benzyl, C₁-C₄ alkylphenyl, C₁-C₄ alkoxyphenyl,halophenyl, C₁-C₄ alkylbenzyl, C₁-C₄ alkoxybenzyl or halobenzyl, 5-, 6-,or 7-membered heterocyclic structure containing two oxygen elements. Thesolution further comprises a compound of Formula II wherein R⁴ is aC₃-C₂₀ linear or branched alkyl. The monomer is electrolyticallydepositing and polymerizing from the solution onto the mixed oxidesurface.

Yet another embodiment is provided in a capacitor. The capacitorcomprises an anode; a dielectric oxide layer coated on the anode and aplurality of conductive islands coated on the dielectric oxide layer. Anorganic conductive cathode is coated on the dielectric layer andconductive islands.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a capacitor of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors of the present application have developed, throughdiligent research, a method of electrodeposition of a conductive polymeron a nonconductive layer comprising a discontinuous seed layer. Theadvantage is provided, in part, by the incorporation of organic sodiumsulfate into the aqueous monomer solution of conductive polymerprecursor. The organic sodium sulfate acts as a surfactant and dopantfor the conductive polymer.

The invention will be described with reference to the FIG. 1 forming apart of the present application.

In FIG. 1, a cross-sectional view of a capacitor is shown as representedat 1. The capacitor comprises an anode, 2, comprising a valve metal asdescribed herein. A dielectric layer, 3, is provided on the surface ofthe anode, 2. The dielectric layer is preferably formed as an oxide ofthe valve metal as further described herein. Coated on the surface ofthe dielectric layer 3, is a discontinuous conductive, or seed, layer,4, preferably comprising manganese dioxide. As realized to one of skillin the art the seed layer facilitates electrodeposition andelectropolymerization of the conducting layer, 5. Leads, 7 and 8,provide contact points for attaching the capacitor to a circuit. Theentire element, except for the terminus of the leads, is then preferablyencased in a housing, 6, which is preferably an epoxy resin housing.

The valve-metal is preferably niobium, aluminum, tantalum, titanium,zirconium, hafnium, or tungsten. Aluminum, tantalum and niobium are mostpreferred. Aluminum is typically employed as a foil while tantalum istypically prepared by pressing tantalum powder and sintering to form acompact. For convenience in handling, the valve metal is typicallyattached to a carrier thereby allowing large numbers of elements to beprocessed at the same time.

The valve metal in the form of a foil is preferably etched to increasethe surface area. Etching is preferably done by immersing the valvemetal into at least one etching bath. Various ethcing baths are taughtin the art and the method used for etching the valve metal is notlimiting herein.

The exterior of the valve metal is coated with a dielectric layercomprising an oxide. It is most desirable that the dielectric layer bean oxide of the valve metal. The oxide is preferably formed by dippingthe valve metal into an electrolyte solution and applying a positivevoltage to the valve metal.

Preferred electrolytes for formation of the oxide on the valve metalinclude aquious solutions of dicarboxylic acids, such as ammoniumadipate. Other materials may be incorporated into the oxide such asphosphates, citrates, etc. to impart thermal stability or chemical orhydration resistance to the oxide layer.

A discontinuous manganese dioxide layer is preferably deposited on thedielectric oxide layer and the conductive polymer layer is formed on thedielectric oxide film layer. The solid electrolyte is preferablyobtained by immersing an anode element in an aqueous manganese nitratesolution. The manganese oxide is then formed by thermally decomposingthe nitrate at a temperature of from 250° to 350° C. in a dry or steamatmosphere. The anode may be treated multiple times to insure optimumcoverage. The manganese dioxide layer forms islands on the surface ofthe anode oxide. In prior art these islands were grown, and additionalislands started, until the islands coalesced into a continuum ofmanganese dioxide. In the instant invention the growth and coalescenceis no longer necessary. With non-continuous surface coatings the organicsulfate allows monomer to be deposited between the islands andelectropolymerization is complete. In practice a seed layer covering 5%of the projected surface area has not been demonstrated to be sufficientfor formation of a continuous polymeric layer. It is preferred that atleast 1% of the projected surface area be covered with a seed layer,preferably manganese dioxide. There is no upper limitation to the amountof surface area above which the invention fails. In instances where acontinuous manganese dioxide layer is present, the invention isoperative even though this represents an inefficient use ofmanufacturing capability. It is preferred that no more than about 80% ofthe projected surface area be covered by a seed layer. More preferablyno more than about 50% and most preferably no more than about 20% of theprojected area is covered by a seed layer.

The conductive layer is formed by dipping the valve metal into anaqueous solution comprising monomer of the conductive polymer and anorganic sulfate. The organic sulfate is believed to act as a dopant forfacilitating conduction and as a surfactant. It is preferred that thesolution be aqueous and most preferably the solution is substantiallyfree of alcohol with less than about 1%, by weight, alcohol in theaqueous solution.

While not restricted to any theory, it is postulated that the organicsulfate provides a sufficient amount of solubilizing characteristic tothe aqueous solution to maintain the monomer in solution. However, thesolubilitization is not sufficient to overcome the desire to precipitateonto the non-conducting surface. This theory is difficult to test due tothe occurrence of foam at higher surfactant levels wherein solubilitycould be sufficient to overcome the precipitation on the surface. Whilereferred to herein as precipitation the result is a sufficientconcentration of monomer at, and between, the islands of manganesedioxide to form a sufficient conducting trace for electropolymerizationto occur. This may be aggregation at, or near, the surface or it may bea liquid solid interface with a concentration gradient of monomertherebetween with the organic sulfate acting as some type at bridgebetween the oxide surface and monomer, or between the manganese oxidelayer and monomer, or between monomer units, or some combination of allof these. The mechanism has not been elucidated and is not limitingherein.

The conducting polymer is preferably the polymer of a monomer whereinthe polymer comprises repeating units of Formula I:

R¹ and R² of Formula I are chosen to prohibit polymerization at theβ-site of the ring. It is most preferred that only α-site polymerizationbe allowed to proceed. Therefore, it is preferred that R¹ and R² are nothydrogen. More preferably R¹ and R² are C-directors. Therefore, etherlinkages are preferable over alkyl linkages. It is most preferred thatthe groups are small to avoid steric interferences. For these reasons R¹and R² taken together as —(CH₂)₂—O— is most preferred.

In Formula 1, X is S, Se or N most preferable X is S.

R¹ and R² independently represent linear or branched C₁-C₆ alkyl orC₂-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzyl which areunsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen orOR³; or R¹ and R², taken together, are linear C₁-C₆ alkylene which isunsubstituted or substituted by C₁₋₆ alkyl, C—C₆ alkoxy, halogen, C₃-C₈cycloalkyl, phenyl, benzyl, C₁-C₄ alkylphenyl, C₁-C₄ alkoxyphenyl,halophenyl, C₁-C₄ alkylbenzyl, C₁-C₄ alkoxybenzyl or halobenzyl, 5-, 6-;or 7-membered heterocyclic structure containing two oxygen elements. R³preferably represents hydrogen, linear or branched C₁-C₁₆ alkyl orC₂-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzyl which areunsubstituted or substituted by C₁-C₆ alkyl.

The organic sulfate is defined by Formula II:R⁴—OSO₃Na  FORMULA II

In Formula II, R⁴ is preferably a C₃-C₂₀ linear or branched alkyl. Belowa C₃ alkyl the sulfate fails to act as a surfactant and the benefits aremitigated. Above about C₂₀ the solubility of the surfactant begins todiminish. More preferably R⁴ is a linear alkyl of 6-20 carbons. Evenmore preferably R⁴ is a linear alkyl of 10-14 carbons. Most preferably,R⁴ is a linear alkyl of 12 carbons wherein the compound is sodiumdodecyl sulfate also referred to as sodium lauryl sulfate.

The aqueous monomer solution preferably comprises about 2% by weight,monomer of Formula I to about 10% by weight, monomer of Formula I. Belowabout 2% by weight, monomer the chemical activity is diminished. Aboveabout 10%, by weight monomer the solubility limit of monomer isapproached thereby increasing the likelihood of premature precipitationand spoilage with minimal increased benefit. More preferable is amonomer concentration of about 4%, by weight, to about 5%, by weight.

The aqueous monomer solution preferably comprises about 4%, by weight,organic sulfate, to no more than about 17%, by weight, organic sulfate.Below about 4% surfactant the benefit is diminished. Above about 17%foam formation initiates which is highly undesirable. The addition oftypical foam inhibitors, such as alcohol, is contrary to one desire andadvantage provided by the invention. About 7.5%, by weight, organicsulfate, has been demonstrated in experiments to be an optional levelwith sufficient surfactant to form a continuous layer of polymer on adiscontinuous seed layer while not approaching the formation of foam.

After contacting the anode with the monomer solution the monomer ispolymerized by electrolytic polymerization. Polymerization is typicallyinitiated at 1-10 volts and 0.01-10 microamps per square centimeter ofsurface area for 5-300 minutes.

The organic sulfate has the benefit of also acting as a dopant in thepolymer. While not necessary, auxiliary dopants can be incorporated astypically employed in the art Specific examples of conducting salts, orauxiliary dopants, for the conductive polymer include but are notlimited to acetates, KHSO₄, Na₂SO₄, HCOOH, LiClO₄, HClO₄, NEt₄ClO₄,NBu₄ClO₄, KAlF₄, NaAlF₄, KBF₄, K₂ZrF₆, KAsF₆, and NaPF₆. Electronacceptor dopants may be used such as NO⁺ and NO₂ ⁺ salts such as NOBF₄,NOPF₄, NOSbF₆, NOAsF₆, NOCH₃SO₃, NO₂BF₄, NO₂ PF₆, NO₂CF₃SO₃. Suitableconducting salts include organic sulfonic acid ions such as aromaticsulfonic acids, aromatic polysulfonic acids, organic sulfonic acids withhydroxy group, organic sulfonic acids with carboxylhydroxyl group,alicyclic sulfonic acids and benzoquinone sulfonic acids, benzenedisulfonic acid, sulfosalicylic acid, sulfoisophthalic acid,camphorsulfonic acid, benzoquinone sulfonic acid, dodecylbenzenesulfonicacid, toluenesulfonic acid. Other suitable dopants include sulfoquinone,anthracenemonosulfonic acid, substituted naphthalenemonosulfonic acid,substituted benzenesulfonic acid or heterocyclic sulfonic acids asexemplified in U.S. Pat. No. 6,381,121 which is included herein byreference thereto. The concentration of dopant, or conducting salt, ispreferably determined such that for every 3 moles of starting monomerthere is present not less than about 1 mole of conducting salt. Theauxiliary dopant can be added in an amount up to about 30%.

Binders and cross-linkers can be incorporated into the conductive layerif desired. Suitable materials include poly(vinyl acetate),polycarbonate, poly(vinyl butyrate), polyacrylates, polymethacrylates,polystyrene, polyacrylonitrile, poly(vinyl chloride), polybutadiene,polyisoprene, polyethers, polyesters, silicones, and pyrrole/acrylate,vinylacetate/acrylate and ethylene/vinyl acetate copolymers. Watersoluble binders such as polyvinyl alcohols are preferred. A solutioncomprising 0 to 30%, by weight binder is preferred.

Carbon paste layers and silver paste layers are formed for attachingelectrode leads as known in the art. The device is then sealed in ahousing.

Other adjuvants, coatings, and related elements can be incorporated intoa capacitor, as known in the art, without diverting from the presentinvention. Mentioned, as a non-limiting summary include, protectivelayers, multiple capacitive levels, terminals, leads, etc.

HYPOTHETICAL EXAMPLE

An aluminum anode can be formed and an aluminum oxide dielectric layerformed thereon as known in the art. The oxidized anode could be placedin a solution comprising manganese nitrate. The oxidized anode couldthen be heated to convert manganese nitrate to manganese dioxide. Theamount of manganese dioxide would be sufficient to form islands coveringapproximately 5% of the projected surface area of the oxidized anode.The coated anode would then be placed in a solution comprising about1-5%, by weight, 3,4-ethylenedioxythiophene, represented by Formula Iwhen X is S and R¹ and R² are taken together to represent —O—CH₂CH₂—O,and about 7.5%, by weight, sodium dodecyl sulfate represented by FormulaII wherein R³ is a C₁₂ straight chain alkyl. Leads and supplementalcoatings can then be incorporated as well known in the art of capacitormanufacture. The capacitor would have excellent capacitance comparableto a prior art capacitor formed with a continuous manganese dioxidelayer and an alcohol based solution.

COMPARATIVE EXAMPLE

A comparative example could be prepared in a manner analogous to theHypothetical Example described above with the exception of pyrrole asthe monomer. The resulting capacitor would most likely fail due to poorstability.

The invention has been described with particular emphasis on thepreferred embodiments. It would be realized from the teachings hereinthat other embodiments, alterations, and configurations could beemployed without departing from the scope of the invention which is morespecifically set forth in the claims which are appended hereto.

1-15. (canceled)
 16. A capacitor formed by the method comprising:forming an oxide layer on a valve metal; contacting said oxide layerwith a solution comprising a monomer precursor of the polymer of FormulaI:

wherein: X is S, Se or N: R¹ and R² are not hydrogen; and a compound ofFormula II:R⁴—OSO₃Na  FORMULA II wherein R⁴ is a C₃-C₂₀ linear or branched alkyl;and polymerizing said monomer. 17-28. (canceled)
 29. A capacitor formedby the method of claim
 17. 30. A capacitor comprising: an anode; adielectric oxide layer coated on said anode; a plurality of conductiveislands coated on said dielectric oxide layer; an organic conductivecathode coated on said dielectric layer and said conductive islands. 31.The capacitor of claim 30 wherein said organic conductive cathodecomprises a polymer formed by the electrolytic polymerization of:

wherein: X is S, Se or N; R¹ and R² are not hydrogen.
 32. The capacitorof claim 30 wherein said organic conductor comprises a dopant.
 33. Thecapacitor of claim 32 wherein said dopant is an organic sulfate.
 34. Thecapacitor of claim 33 wherein said dopant is:R⁴—OSO₃Na wherein R⁴ is a C₃-C₂₀ linear or branched alkyl.
 35. Thecapacitor of claim 31 wherein X is S.
 36. The capacitor of claim 31wherein R¹ and R² independently represent linear or branched C₁-C₁₆alkyl or C₂-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzylwhich are unsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy,halogen or OR³; or R¹ and R², taken together, are linear C₁-C₆ alkylenewhich is unsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy,halogen, C₃-C₈ cycloalkyl, phenyl, benzyl, C₁-C₄ alkylphenyl, C₁-C₄alkoxyphenyl, halophenyl, C₁-C₄ alkylbenzyl, C₁-C₄ alkoxybenzyl orhalobenzyl, 5-, 6-, or 7-membered heterocyclic structure containing twooxygen elements. R³ preferably represents hydrogen, linear or branchedC₁-C₁₆ alkyl or C₂-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl orbenzyl which are unsubstituted or substituted by C₁-C₆ alkyl.
 37. Thecapacitor of claim 36 wherein said R¹ and R² are taken together to formO—CH₂ CH₂—O.
 38. The capacitor of claim 30 wherein said valve metal isselected from a group consisting of niobium, aluminum, tantalum,titanium, zirconium, hafnium and tungsten.
 39. The capacitor of claim 38wherein said valve metal is selected from a group consisting of niobium,aluminum and tantalum.